genhd: Fix use after free in __blkdev_get()

When two blkdev_open() calls race with device removal and recreation,
__blkdev_get() can use looked up gendisk after it is freed:

CPU0				CPU1			CPU2
							del_gendisk(disk);
							  bdev_unhash_inode(inode);
blkdev_open()			blkdev_open()
  bdev = bd_acquire(inode);
    - creates and returns new inode
				  bdev = bd_acquire(inode);
				    - returns the same inode
  __blkdev_get(devt)		  __blkdev_get(devt)
    disk = get_gendisk(devt);
      - got structure of device going away
							<finish device removal>
							<new device gets
							 created under the same
							 device number>
				  disk = get_gendisk(devt);
				    - got new device structure
				  if (!bdev->bd_openers) {
				    does the first open
				  }
    if (!bdev->bd_openers)
      - false
    } else {
      put_disk_and_module(disk)
        - remember this was old device - this was last ref and disk is
          now freed
    }
    disk_unblock_events(disk); -> oops

Fix the problem by making sure we drop reference to disk in
__blkdev_get() only after we are really done with it.

Reported-by: Hou Tao <houtao1@huawei.com>
Tested-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>